Bone Anchor With Locking Cap and Method of Spinal Fixation

ABSTRACT

A bone anchor ( 100 ) for attaching a rod ( 108 ) to a bone has an anchor member ( 106 ) for attachment to the bone and an anchor head ( 104 ) having a U-shaped opening for receiving the rod. The bone anchor also includes a locking cap ( 102 ) that has a main body ( 900 ) and a set screw ( 1000 ). Advantageously, in one embodiment the locking cap preferably is designed such that a single tool can be used to lock the locking cap in place on the anchor body preferably with a 90° turn and preferably then drive the set screw to clamp the rod. The locking cap also preferably non-threadingly engages the anchor body. The anchor body preferably has an inclined surface on its top surface which elastically deflects extending tabs on the main body to secure the locking cap to the anchor body. A method of implantation and assembly of the bone anchor are also described.

TECHNICAL FIELD OF THE INVENTION

This invention relates to bone fixation devices and related methods offixation. More specifically, this invention relates to bone anchors,such as screws and hooks for spinal fixation including polyaxial boneanchors, and related methods of spinal fixation.

BACKGROUND OF THE INVENTION

Many methods of treating spinal disorders are known in the art. Oneknown method involves anchoring a screw or a hook to the vertebrae, andfixing the screws or hooks along a spinal rod to position or immobilizethe vertebrae with respect to one another. The screws or hooks commonlyhave heads with U-shaped channels in which the spinal rod is insertedand subsequently clamped by a set screw or other fastener or lockingcap. This method may commonly involve multiple screws or hooks, as wellas multiple spinal rods. With this method, the spinal rod(s) may beshaped to maintain the vertebrae in such an orientation as to correctthe spinal disorder at hand (e.g., to straighten a spine having abnormalcurvature). Additionally or alternatively, the screws or hooks may bespaced along the rods(s) to compress or distract adjacent vertebrae.

Surgeons have often encountered considerable difficulty with this methodbecause of trouble aligning the spinal rod(s) with the U-shaped channelsin the heads of the screws or hooks. For example, the heads of thescrews or hooks are often out of alignment with one another because ofthe curvature of the spine or the size and shape of each vertebrae. Tofacilitate easier insertion of the spinal rods into the U-shapedchannels, and to provide additional flexibility in the positioning ofthe spinal rods and the screws and hooks, screws and hooks have beendeveloped wherein the head (which includes the U-shaped channel)initially pivots with respect to the anchor member (e.g., screw shank orhook). One example of such a screw is disclosed in U.S. Pat. No.5,586,984 to Errico et al., which is incorporated herein by reference.

The process of positioning and setting known bone anchors may be tediousand relatively time consuming, typically requiring more than onesurgical tool to clamp the spinal rod and anchor member in desiredpositions. Even with a high degree of skill and care, the process ofpositioning a known set screw or other fastener on the bone anchor andthen manipulating the screw or fastener to clamp or re-clamp the spinalrod and anchor member in place can take more time than desired during asurgical procedure or even resulting in the rod, anchor member, or bothmoving out of position before clamping is completed.

Therefore, a need still exists for bone anchors including polyaxial boneanchors that provide an improved mechanism for clamping the spinal rodand anchor member in their desired positions.

SUMMARY OF THE INVENTION

The invention is directed to a bone anchor including a polyaxial ormonoaxial bone anchor for attaching a rod, such as a spinal rod or otherdevice, to a bone, such as a vertebra. The bone anchor may include ananchor member (such as a screw, hook, or similar structure) forattachment to the bone and an anchor head of the bone anchor. The anchorhead may be integral with the anchor member or may be a separate pieceor component that couples with, and may polyaxially rotate about, theanchor member. The anchor head preferably has U-shaped openings forreceiving the rod. The anchor head may have a recess for receiving thehead of the anchor member such that the anchor member can initiallypolyaxially angulate or rotate with respect to the anchor head. Thepolyaxial bone anchor may also include a locking cap capable of clampingthe rod and anchor member in desired positions.

The locking cap advantageously simplifies and preferably shortens theclamping process. The locking cap preferably includes a main body, a setscrew, and a saddle. The set screw is preferably preloaded into the mainbody so that upon application of a torque, preferably below apredetermined amount, for example about 4.5 Newton-meters, the main bodyrotates with the set screw. Preferably, the saddle is in contact with orcoupled to the set screw. Typically, after the anchor member isimplanted into a bone and a rod is inserted into the bone anchor, thelocking cap is positioned on top of the anchor head where features ofthe main body engage features of the head to allow the locking cappreferably to lock into place on the anchor head preferably with about a90° turn or less. The instrument preferably engages the set screw andapplication of a rotary motion in a direction to lock the cap to thebody (anchor head) also rotates the main body so that the main body iscoupled to the bone anchor. Once the rod and anchor member are alignedas desired, the set screw can then be twisted into the anchor head,which causes the saddle to engage and then clamp the rod in place. As aresult, the clamping of the rod may cause the anchor member in apolyaxial bone anchor to be clamped in place. Alternatively the anchormember may be locked to the anchor head in a polyaxial bone anchorindependently of the rod to be clamped to the bone anchor.Advantageously, features of the main body and set screw allow the sametool to be used in a single action to position and lock the main bodyinto the anchor head and to then rotate the set screw.

A bone anchor for attaching a fixation element to a bone according toone embodiment includes an anchor member (e.g., a bone screw) forattachment to said bone; a body having a top portion and a bottomportion, the body coupled to the anchor member proximate the bottomportion, the body forming a hollow interior and having a generallyU-shaped channel in said top portion for receiving said fixationelement; and a locking cap removably, non-threadably mountable to saidtop portion of said anchor head, the locking cap may have a main body(sometimes referred to as a locking ring) and a set screw threadablyattached to the main body to engage and clamp the fixation element tothe body. The main body of the locking cap may have one or moreresiliently deflectable tabs and the body may have an inclined surfaceon its top surface, wherein the tab deflects over the inclined surfaceon the body upon rotation of the main body with respect to the body.

The main body and the set screw may be preloaded such that rotation ofthe set screw causes the main body to rotate. The set screw may bepreloaded with the main body such that a force of about 2 to about 6Newton-meters is necessary to allow the set screw to rotate with respectto the main body. The main body and set screw may be driven by a singleinstrument engaging only said set screw until said main body is lockedin place on said housing. The body may further include a stop surface onits top surface to resist the main body from being removed from thebody. The top surface of the body may further have a retention surface,and the stop surface and the retention surface may form a recess toretain the tab of the main body. The main body may rotate about 90° orless with respect to the body to secure the main body to the body. Themain body of the locking cap may include one or more dovetail-shapedprojections and the housing may include one or more dovetail-shapedgrooves in the top portion. When the main body attaches to the body, thedovetail projections are received in the dovetail grooves to preventsplaying but the dovetail connection in some embodiments may not providea force on the body during rotation of the main body. The locking capfurther may include a saddle coupled to the set screw and independentlyrotatable with respect to the set screw return.

The bone anchor may optionally include a compressible collet having abottom surface abutting a lower flange in the bottom portion of thebody. The collet may have an extended shoulder that rests on the lowerflange of the body. The bone anchor may further optionally include asleeve having an internal recess that receives the collet and has atapered inner surface to provide a compression force on the externaltapered surface of the collet upon movement of the sleeve toward thebottom portion of the body to lock the anchor member. The sleevepreferably has a channel to receive the fixation element and the channelhas at least a portion that has a smaller opening than the fixationelement so that the fixation element is snap-fitted into the sleeve. Thechannel of the sleeve has an opening and the sleeve further has a topsurface that has an inclined surface that interacts with a bottomsurface of the locking cap to spread the sleeve opening. The body mayhave an assembly groove and the collet may have an extended shoulderwhere the assembly groove is configured to permit the bottom opening ofthe collet to expand to receive the head of the anchor member byreceiving the extended shoulder.

A bone anchor for attaching a fixation element to a vertebrae accordingto another embodiment comprises an anchor member for attachment to saidvertebrae; a body having a top portion and a bottom portion, the bodyforming a generally tubular-shaped element having an interior cavity, atop opening in the top portion communicating with the interior cavityand a bottom opening in the bottom portion communicating with theinterior cavity, the top portion having an end wall, at least one grooveand channel to receive the fixation element, the end wall has one ormore dimples formed by an inclined surface and a straight wall, theinclined surface being inclined with respect to the end wall and thestraight wall being substantially perpendicular with respect to the endwall; and a locking cap removably mountable to the top portion of thebody, the locking cap comprising a locking ring and a set screwthreadably attachable to the locking ring, the locking ring comprisingone or more deflectable tabs and one or more projections, thedeflectable tab is configured and adapted to interact with itsrespective dimple to deflect as the locking ring is rotated with respectto the body and the projection is configured and adapted to be receivedin its respective groove upon rotation of the locking ring. The end wallmay further include an abutment surface, the abutment surface and thestraight wall forming a recess for the deflecting tab upon rotation ofthe locking ring approximately 90°. The projection may be dovetailshaped and the groove may be dovetail shaped and wherein the respectivedovetail shapes are adapted and configured so as not to provideengagement forces as the dovetail projection is received in the dovetailgroove but the interaction of the projection and the groove may preventthe top portion from expanding beyond a predetermined distance.

The bone anchor may comprise a sleeve having a U-shaped channel adaptedand configured to receive the fixation element, retention tabs, and abottom portion having an internally tapered surface; and a compressiblecollet having an externally tapered surface, a hollow interior toreceive a portion of the anchor member, a bottom opening, a plurality offlexible fingers and a shoulder extending beyond the external surface,the bottom opening of the collet configured and adapted in its naturalstate to be smaller than the portion of the anchor member received inthe interior of the collet and having an expanded state that allows atleast a portion of the anchor member to be moved therethrough; whereinthe body comprises a flange proximate the bottom opening, an assemblygroove in the bottom portion, and sleeve retention slots, the bodycontaining the collet proximate the flange and the sleeve internallytapered surface contacting the collet external surface; wherebytranslating movement of the sleeve within the body compresses theresilient fingers of the collet inward to fix the anchor member relativeto the head without the collet undergoing translating movement. Theanchor member may be adapted and configured to be insertable into thebody through the bottom opening and the collet may be adapted andconfigured to be insertable into the body through the top opening of thebody but not the bottom opening, and wherein the anchor member may beinsertable into the collet only when the shoulder of the collet isreceived in the assembly groove. The retention tabs of the sleeve may bepositioned in slots formed in the body. The slots and retention tabs maybe configured and adapted to align the fixation element channel of thesleeve with the fixation element channel of the body.

A method of manufacturing a bone anchor for a spinal fixation devicethat receives a fixation element may include the steps of providing ananchor member having a bone engagement portion and a head; providing abody having a top opening, a bottom opening, an interior cavitycommunicating with the top and bottom openings, a channel to receive thefixation element and an assembly groove, the bottom opening beingsmaller than the top opening; providing a collet having a bottom openingcommunicating with an interior cavity, and a shoulder proximate thebottom opening, the interior collet cavity being configured to receiveat least a portion of the head of the anchor member and the bottomopening being smaller than at least a portion of the anchor member head;inserting the head of the anchor member through the bottom opening ofthe body; aligning the shoulder of the collet with the assembly grooveof the body; inserting the head of the anchor member through the bottomopening of the collet while the collet is located in the interior cavityof the body and the shoulder is aligned with the assembly groove so thatthe collet expands and the shoulder is received into the assemblygroove; forming a collet and anchor member assembly by allowing thecollet to retract so that the shoulder is moved out of the assemblygroove and the head of the anchor member is retained by the collet;repositioning the collet and anchor member assembly so that the shoulderis not aligned with the assembly groove. The method of assembly mayfurther comprise providing a sleeve having an internally taperedsurface, retention tabs and a channel to receive the fixation element;inserting the sleeve into the interior cavity of the body through thetop opening; and positioning the sleeve so that at least a portion ofthe internally tapered surface overlies at least a portion of theexterior surface of the collet and so that the retention tabs snap intoslots formed in the body after the collet anchor member assembly isformed in the body.

A method of fixing the vertebrae of the spine using a first bone anchorhaving a first anchor member and a first anchor head having a firstrod-receiving opening or channel, and a second polyaxial bone anchorhaving a second anchor member and a second anchor head having a secondrod-receiving opening or channel is also described. The method mayinclude inserting the first anchor member into a first vertebra andinserting the second anchor member into a second vertebra. The firstanchor member may be inserted into a lateral mass of a first vertebra,and the second anchor member may be inserted into a lateral mass of asecond vertebra. As an example, at least one of the first and secondvertebrae preferably may be selected from the group of vertebraecomprising the lumbar or thoracic regions of the spine although otherareas of the spine are contemplated.

After the bone anchor is inserted in the bone, a rod may be insertedinto the rod receiving channel from the top of the bone anchor. Thelocking cap may then be placed on the top of the bone anchor and rotated90° or less to engage the locking cap with the bone anchor, and inparticular a main body of the locking cap engaging the anchor head ofthe bone anchor. With the locking cap in this position, the rod ispositioned in the rod receiving channel of the bone anchor and mayundergo translating and/or rotating movement in the rod receivingchannel. Using the same tool that was used to twist the main body toengage the main body to the anchor head, the set screw may then berotated upon application of a predetermined torque to the set screw sothat the set screw can rotate independently of the main body. Furtherrotation of the set screw clamps the rod in position, and may furtherclamp the position of the anchor head or body to the anchor member,which may be, for example, a screw or hook. Alternatively, with thelocking cap engaged to and covering the opening of the anchor body, theposition of the anchor head to the anchor member may be clamped andfixed, although the spinal rod may undergo rotational or translatingmovement in the rod receiving channel.

The method may also include coupling the rod to a bone anchor by lockingthe main body to the anchor head, positioning the rod relative to one orboth of the bone anchors, for example, by rotating or translating therod in one or more of the rod receiving channels and clamping the rod inone or more of the bone anchors, for example, by rotating the set screwof a respective bone anchor.

The method may further include loosening the set screw so that the rodcan be moved relative to the bone anchor by, for example, rotating theset screw in an opposite or second direction, repositioning the rod inone or more of the rod receiving channels, and/or repositioning theanchor head relative to the anchor member of one or more of the boneanchors, and relocking/clamping the rod into position and/orrelocking/clamping the anchor member relative to the anchor head.

The method may still further include rotating the locking cap in anopposite or second direction in order to loosen or remove the lockingcap from the bone anchor, and more particularly rotating the main bodyin an opposite or second direction to disengage and, if desired, removethe locking cap from the bone anchor. The main body may be rotated byusing the same first tool used to couple the main body to the anchorhead and to tighten the set screw, or alternatively a second tool may beused to loosen the main body, or alternatively both the first and secondtools may be used to loosen the main body from the anchor head.

BRIEF DESCRIPTION OF THE DRAWINGS

The detailed description will be better understood in conjunction withthe accompanying drawings, in which like reference characters representlike elements, as follows:

FIGS. 1-6 are perspective, front, side, front cross-sectional (takenalong line A-A of FIG. 6), side cross-sectional (taken along line B-B ofFIG. 6), and top views, respectively, of an illustrative embodiment of apolyaxial bone anchor according to the invention;

FIG. 7 is front cross-sectional view (taken along line A-A of FIG. 6) ofthe anchor head and locking cap of the polyaxial bone anchor of FIGS.1-6;

FIG. 7A is a perspective view of the anchor head without the lockingcap;

FIGS. 7B1-5 are front, side, top, sectional, and partial cross-sectionalviews, respectively, of an embodiment of a sleeve showing representativedimensions.

FIG. 8 is a perspective view of an embodiment of a slotted colletaccording to the invention;

FIG. 9 is a perspective view of the main body of a locking cap accordingto the invention;

FIG. 10 is a perspective view of the set screw of a locking capaccording to the invention;

FIG. 11 is a perspective view of the saddle of a locking cap accordingto the invention;

FIGS. 12-15 are top, front cross-sectional (taken along line A-A of FIG.12), side cross-sectional (taken along line B-B of FIG. 12), andenlarged partial cross-sectional views, respectively, of the saddle ofFIG. 11 showing representative dimensions;

FIGS. 16-21 are perspective, top, front, front cross-sectional (takenalong line A-A of FIG. 17), side, and side cross-sectional (taken alongline B-B of FIG. 17) views, respectively, of the assembled locking capcomprising the components of FIGS. 9-11;

FIGS. 22-26 are perspective, top, front cross-sectional (taken alongline A-A of FIG. 23), and two enlarged partial cross-sectional views,respectively, of the polyaxial bone anchor with the locking cap insertedand in an open, nonengaged or uncoupled position and with the rodunlocked;

FIG. 27 is a top view of the locking cap inserted and in an openposition on the anchor head;

FIGS. 28-32 are perspective, top, front cross-sectional (taken alongline A-A of FIG. 29), and two enlarged partial cross-sectional views,respectively, of the polyaxial bone anchor with the locking cap insertedand in a closed, engaged or coupled position and with the rod unlocked;

FIG. 33 is a top view of the locking cap inserted and in a closedposition on the anchor head;

FIGS. 34-38 are perspective, top, front cross-sectional (taken alongline A-A of FIG. 35), and two enlarged partial cross-sectional views,respectively, of the polyaxial bone anchor with the locking cap insertedand in a closed, engaged or coupled position and with the rod locked;and

FIGS. 39 and 40 are perspective views of illustrative embodiments of amonoaxial bone anchor and a spinal hook employing a locking cap.

DETAILED DESCRIPTION OF THE INVENTION

While the bone anchor will be illustrated and described herein withreference to certain preferred or exemplary embodiments, the inventionshould not be limited to these preferred or exemplary embodiments.Furthermore, the features described and illustrated herein can be usedsingularly or in combination with other features and embodiments.

FIGS. 1-6 show an embodiment of a polyaxial bone anchor. Polyaxial boneanchor 100 includes a locking cap 102 (not shown in FIGS. 2-5), ananchor head 104, and an anchor member 106. Bone anchor 100 may also be amonoaxial bone anchor such that the anchor member and anchor head areintegral members that are fixed together. (See FIG. 39). Anchor head 104(sometimes referred to in the art as the body) has a generally U-shapedopening 101 for receiving a spinal rod 108 (note that spinal rod 108 isnot shown in FIGS. 2-6) or other device, such as, for example, a plate.Anchor member 106, which may be a bone screw, hook, or other similarstructure, is coupled to anchor head 104 such that it can preferablypolyaxially rotate with respect to head 104. In one embodiment of theinvention, anchor member 106 can angularly rotate about axis 107 by anangle θ of preferably at least about 25° in any direction (i.e., theangular rotation of anchor member 106 forms a cone of about 50°),although other ranges of angulation are contemplated. One or morepolyaxial bone anchors 100 may be attached, for example, to thevertebrae via respective anchor members 106, and a spinal rod 108 orother device can be inserted into the U-shaped openings 101 andthereafter locked to correctly align the spine or treat other spinaldisorders.

As shown in FIGS. 7 and 7A, anchor head 104 includes a generallycylindrical, hollow housing 710, and may optionally contain a generallyU-shaped sleeve 612, and a collet 814. Housing 710 has generallycylindrical, untapered interior wall portions 711 a,b, but alternativelycan have at least some wall portions that are tapered inward towardanchor member 106. The exterior shape of housing 104 may be generallycylindrical and may have tapered or straight wall portions. Otherexterior shapes for housing 104 are also possible. Sleeve 612 hasretention tabs 613 a,b that snap into respective slots 715 a,b inopposite walls of housing 710 as sleeve 612 is inserted preferablydownward through housing 710. In one embodiment of the invention, slots715 a,b are sized to allow sleeve 612 to move up and down about 1.5 mm,although other distances are contemplated. One or more slots may beformed in housing 710 depending upon the number of retention tabs. Theremay be more slots formed than retention tabs. Moreover, the slots canextend or be formed across the width or circumference of the interiorhousing wall so that the sleeve can rotate (spin) in the housing or theslots can be formed to be approximately the same width as the tabs sothat the sleeve cannot rotate (spin) in the body. The configuration andinteraction of the retention tabs on the sleeve and slots can bedesigned so that the sleeve will be aligned in the body, or so that noalignment feature is provided. Accordingly, the retention tabs caninteract with the slots to align the fixation element channel of thesleeve with the fixation element channel of the body. The retention tabsand slots may be reversed so that the tabs may be formed on the housingand the slots may be formed on the sleeve. Anchor head 104 has a topportion 114 and a bottom portion 118 and a bore 116 extending alonglongitudinal axis 107. A top opening 414, bottom opening 415 andU-shaped opening 101 communicate with bore 116. Preferably a flange 718extends inward proximate bottom opening 415. Flange 718 preferably formsa substantially flat interior surface 720 that is preferablysubstantially perpendicular to longitudinal axis 107. Surface 720 mayalso be curved, and may be inclined at non-perpendicular angles withrespect to axis 107.

Anchor member 106 preferably has a curved head 717 which is shaped anddimensioned to fit within housing 710 and an internal space 815 ofcollet 714 (note that the top portion of curved head 717 is cut away inFIG. 7 to show collet 714). Curved head 717 may be substantiallyspherical or frustospherical and preferably has a recess 112 that iskeyed to receive a hex wrench, torque wrench, or other known driver toallow anchor member 106 to be implanted by rotating into, for example, avertebra.

FIGS. 7B1-5 show various views of a sleeve 612. Sleeve 612 preferablyhas a U-shaped channel 610 to receive spinal rod 108 or some otherfixation element. Channel 610 is formed by stanchions 615 a and 615 b.Sleeve 612 is generally cylindrically shaped, although other shapes arepossible, and has a longitudinal bore 616. The stanchions 615 a,b formedon the top part of the sleeve are flexible and preferably include theretention tabs 613 a,b. Instead of two retention tabs, one or moreretention tabs may be utilized. A top opening 618 communicates with thebore 616 and the channel 610. The stanchions 615 a,b form partialcylindrical or curved walls. The width of U-shaped openings 610 at thetop opening 618, formed by the outer edges of the curved wall of thestanchions 615 a,b, preferably are smaller than the diameter or width ofthe spinal rod 108 so that the rod flexes the stanchions upon insertioninto the sleeve and snaps into place in the sleeve. The top innersurface 622 has a taper or incline at the top opening 618. The smalleropening 610 formed by the flexible stanchions permits the stanchions 615a,b to flex, compress and grip the rod when it is inserted into theU-shaped channel 610 of the sleeve 612. The tapered top inner surface622 may facilitate the spreading of the stanchions 615 a,b.

A recess 625 is formed in the lower or bottom portion 626 of the sleeve612. The exterior surface 628 of the bottom portion may be straight andgenerally cylindrically shaped. The recess 625 communicates with bottomopening 630 and preferably with bore 616. The interior walls formingrecess 625 preferably are at least partially tapered as show in FIG. 7B1. The angle of the taper is preferably about 8° although other anglesare contemplated. The recess 625 as described below receives the collet814.

The diameter of the sleeve 612 is preferably about 9 mm to about 10 mmalthough other diameters are contemplated. The width or length of theopenings 610 formed by the stanchions 615 a,b at its smallest width ispreferably about 5.9 mm (for a 6.0 mm spinal rod), although otherdimensions are contemplated. The height of the sleeve 612 is preferablyapproximately about 9 mm to about 10 mm although other dimensions arecontemplated. Alternatively, sleeve 612B may be of other dimensions.

FIG. 8 shows an embodiment of a collet 814. The collet preferably hasslots 806 to create resilient fingers 810 that can deflect outward toallow head 717 of the anchor member to be inserted within the internalspace 815 formed by the collet. The internal space 815 is formed by abore 850 that communicates with top opening 852 and bottom opening 854.Slots 806 may also have a radius or circular shaped portion 808 as astress relief and to provide better resiliency to fingers 810. Althoughthe slots are shown as extending from the lower or bottom end 812 intowhich head 717 of the anchor member is inserted, the slots canalternatively extend from the top end 820, or alternatively slots 806can extend from both bottom end 812 and top end 820. In one embodiment,collet 814 may have a slot that extends from the top end 810 through tobottom end 812 so that it may form a C-ring. The arrangement, shapes,and dimensions of the slots may be alternatively different than shown.The bottom opening 854 preferably is sized so that it is smaller thanthe maximum diameter of the head 717 of the anchor member. The internalspace 815 preferably has a larger diameter than the bottom opening 854to accommodate.

Collet 814 preferably has an externally tapered surface 822. Theexterior tapered surface 822 preferably is about 8° and preferablysubstantially matches the taper of the interior surface of the recessarea 625 of the sleeve 612 to form what is sometimes referred to as aMorse Taper. The collet also has an interior surface 824 preferablyshaped to substantially match the shape of head 717. Preferably theinterior surface 824 is semi-spherically shaped to match the shape ofhead 717, although surface 824 may also be conically shaped or haveother shapes. Collet 814 is made of a resilient material, preferablytitanium alloy, that can be compressed around head 717 to retain anchormember 106 securely in place. Preferably the material of the collet issofter than the material of sleeve 612 and anchor member 106. In oneembodiment of collet 814, top inside diameter 816 is about 6.3 mm andheight 818 is about 3.7 mm. Alternatively, collet 814 may be of otherdimensions.

In a preferred embodiment, collet 814 may have a ring 826 forming anextended shoulder 828 around the bottom end 812. Preferably an undercutradius 827 is formed at the exterior surface where the tapered exteriorsurface 822 meets the extended shoulder 828. The interior surface 830(FIG. 25) opposite the extended shoulder 828 preferably is flat andsubstantially perpendicular to the substantially flat bottom surface832, although surface 830 may be curved and non-perpendicular to bottomsurface 832. The bottom ring 826 preferably provides increased surfacearea at bottom surface 832 to rest on the flange 718 formed proximatethe bottom opening 415 of the housing 710. The extended shoulder 828 mayprovide extra rigidity to the collet 814 and may better retain the boneanchor in the housing 710.

Collet 814 is preferably inserted into top end 114, or the U-shaped rodreceiving channel 101, and is sized so that it can be inserted into bore116 of the anchor head from top 114, but is also sized and dimensionedso that it is retained in the lower end of bore 116 of the anchor head.In other words, the diameter of the collet is such that it can passthrough the top opening 414 of bore 116, but preferably cannot passthrough the bottom opening 415 of bore 116. Alternatively, fingers 810of the collet can be compressed to allow the collet to be insertedthrough bottom opening 415, and once within anchor head 104, the fingerscan expand. Thus, the collet can be inserted into the top or bottomopening of the anchor head. After the collet is inserted, sleeve 612 maybe inserted into the top opening or from the side through the U-shapedrod receiving channel 101 so that the lower end 626 of the sleeve ispositioned around top end 820 of the collet. Sleeve 612 may be presseddown until retention tabs 613 a,b snap in retention slots 715 a,b.Anchor member 106 may be inserted from bottom opening 415 of bore 116.Collet 814 may then be inserted down bore 116 and snapped over head 717of anchor member 106. Alternatively, collet 814, or collet 814 andsleeve 712 may be inserted and positioned in anchor head 104 and head717 of anchor member 106 inserted into bottom opening 414 and snappedinto the collet. When head 717 is inserted into the bottom opening 812of the collet 814, the sleeve is positioned to allow the collet toexpand so that the head may pass through bottom opening 812 andthereafter the collet would contract to hold the head 717 of the anchormember 106.

If the collet 814 with the extended shoulder 828 is used, the body orhousing 710 preferably has an assembly groove 712 preferably formed inthe bottom portion 118 of the housing 710. Preferably the assemblygroove is formed in the interior walls of the housing 710 between theflange 718 and the sleeve retention slots 715 a,b, although otherlocations for the assembly groove are contemplated. The assembly groove712 permits the bottom opening 854 of the collet 814 to expand while itis inside the bore 116 of the housing 710 to permit the head of theanchor member to pass through the smaller opening 854 of the collet 814.

In a preferred assembly method, the collet 814 is inserted into the topopening 414 of the housing 710. The head 717 of the anchor member 106 isinserted through the bottom opening 415 of the housing. With theextended shoulder 828 of the collet aligned with the assembly groove 712in the housing 710, the head 717 of the anchor member is insertedthrough the bottom opening 854 of the collet. Since the bottom opening854 is smaller than the head 717 of the bone anchor 106, the colletexpands and the extended shoulder 828 is temporarily received in theassembly groove 712 until the maximum diameter of the head 717 passesthrough the opening 854. As the head 717 is inserted further into theinternal space 815, the fingers 810 retract and the extended shoulder828 moves out of the assembly groove. Thereafter the anchor member 106and collet 814 assembly are moved so that the extended shoulder 828 isno longer aligned with the assembly groove 712 and the anchor member isretained by the collet such that the anchor member can angulate androtate within the collet but cannot be released from the collet becausethe collet cannot expand sufficiently to permit the head 717 to passthrough the bottom opening 854. Preferably the collet and anchor memberassembly are moved so that the bottom surface 832 of the collet contactsand is retained by the flange 718 of the housing 710.

With the collet and bone member assembly in the housing 710, the sleeve612 may be inserted into the top opening 424 of the housing and moveddown so that the recess 625 receives and overlaps with a portion of thecollet 814. The sleeve is inserted in the housing 710 preferably so thatthe retaining tabs 613 a,b are received in the retaining slots 715 a,b.In this manner, the sleeve is retained in the housing 710 and in turnretains the collet 814 and the anchor member 106 in the housing. Morepreferably, the sleeve is moved down into housing 710 so that itsinternal tapered surface 622 overlaps with the external tapered surface822 of the collet 814 when the retention tabs 613,ab are received in theretaining slots 710 a,b. With the sleeve inserted, the anchor memberpreferably is still permitted to polyaxial angulate and rotate aboutlongitudinal axis 107 of housing 710.

FIG. 7 further shows locking cap 102 locked into anchor head 104.Locking cap 102 preferably includes a main body or locking ring 900(also shown in FIG. 9), a set screw 1000 (also shown in FIG. 10), and asaddle 1100 (also shown in FIG. 11). In one embodiment, main body 900preferably has an outside diameter 919, measured from tab 920 a to tab920 b, of about 12.5 mm, and set screw 1000 preferably has a height 1018of about 4.5 mm and preferably an outside diameter 1019 of about 7.0 mm.Alternatively, main body 900 and set screw 1000 may be of otherdimensions. FIGS. 13-14 show various views of saddle 1100.Alternatively, saddle 1100 may be of other dimensions.

FIGS. 16-21 show various views of locking cap 102. Set screw 1000 hasexternal threads 1023 that mate with internal threads 921 of main body900. Advantageously, set screw 1000 cannot be screwed out of main body900 in the direction of up arrow 1824 in FIG. 18 because of flaredportion 1925 at the bottom of set screw 1000. Set screw 1000 ispreferably preloaded into main body 900 before locking cap 102 is usedin a bone anchor. More specifically, set screw 11000 is rotated in thedirection of arrow 1824 (in FIG. 18) so that portion 1925 engages bottomend 1926 of the internal threads of main body 900 and tightened so thata friction fit is created. The friction fit is such that it takes acertain predetermined torque ST applied to the set screw to free the setscrew from main body 900. Until the predetermined torque ST is applied,any rotational torque applied to the set screw rotates the set screw andthe main body. The torque to free the set screw preferably may be about2 to about 6 Newton-meters, and more preferably about 4.5 Newton-meters.In one embodiment, set screw 1000 has a star socket 1727. Alternatively,set screw 1000 can have other types of sockets or recesses keyed toother known drivers or tools. Advantageously, as described in moredetail below, a single tool or drive mechanism can drive in a singleaction both main body 900 and set screw 1000 simultaneously to locklocking cap 102 in place on an anchor head and then continue driving setscrew 1000 alone until a rod inserted in the bone anchor and an anchormember are clamped in place.

Saddle 1100 is preferably coupled to set screw 1000. That is, set screw1000 is fitted onto upper portion 1122 (see FIG. 11) of saddle 1100, theupper portion 1122 preferably being cylindrically shaped, and then thetop of portion 1122 is splayed slightly to create a flange. Moreparticularly, upper portion 1122 forms a stem 1130 which is fittedthrough a bore 1020 that communicates with recess 1727 in the set screw1000. After the top portion 1132 of stem 1130 protrudes into the recess1727, the top portion where a wall has been formed by depression 1136 isflared outward to couple the saddle 1100 to the set screw 1000. Thesaddle is coupled to set screw 1000, but is free to rotate with respectto et screw 1000 about an axis 1310 (see, e.g., FIG. 19). Stem 1130 hasa tapered transition region 1135 where it joins lower portion 2129. Thebottom opening of bore 1020 has a tapered recess region to accommodatetransition region 1135. Saddle 1100 has a recess 1320 which has a radiusof curvature about the same as the spinal rod.

Saddle 1100 preferably has helical grooves 2130 on lower portion 2129(see FIG. 21) to enhance gripping of the rod. Preferably the helicalgrooves are inclined, similar to screw threading, and thus notperpendicular to the axis formed by the recess 1320. The groovesgenerally run in the direction of arrow 2135 (shown in FIG. 13A) fromextending portion 1142 to extending portion 1144. The helical groovespreferably are only slightly inclined and may provide increased clampingstrength on the rod as they are slightly offset from being perpendicularto the longitudinal axis of the rod.

Housing 710 has an end wall or top surface 730 that preferably has oneor more of the following features to assist in cooperating with thelocking cap 102 to attach the locking cap to the bone member. The topsurface 730 (also referred to as the end wall) preferably may havedimples 2730 a,b formed by respective ramped surfaces 2735 a,b andstraight-walled stop surfaces 2737 a,b. Top surface 730 may also containretention surfaces 2738 a,b as well as structures 733 a,b. The outeredge 735 of the top surface 730 may be beveled or inclined. The mainbody 900 may have extending tabs 920 a,b and straight surface 925 a,b.The extending tabs 920 a,b form abutment surfaces 930 a,b. Preferablythe abutment surfaces 930 a,b are substantially perpendicular to thestraight surfaces 925 a,b, although they may be non-perpendicular. Theouter edge of the tabs 920 may also have a beveled or inclined surface.Main body 900 also preferably has flanges 3236 and 3238 and extendingsurface 2632. Flanges 3236 and 3238 will interact with grooves 2636 and2638 respectively formed in housing 710. The interaction of thesestructures of the housing with the main body 90 will be described inmore detail below.

To lock (i.e., clamp) a rod and anchor member in place in a bone anchor,locking cap 102 is positioned on top of housing 710 as shown in FIGS.22-27. Locking cap 102 and particularly tabs 920 a,b are oriented withrespect to dimples 2730 a,b on the top surface 730 of housing 710 asshown in FIG. 27. Preferably surface 2632 of main body 900 (best seen inFIG. 26) extends slightly beyond surface 2634 of housing 710 when mainbody 900 is oriented in housing 710 as shown in FIGS. 22-27. As suchlocking cap 102 is pressed downward until it snaps into the positionshown in FIGS. 22-27. Note that ramp surface 2642 on housing 710 andramp surface 2644 on main housing 900 facilitate this snap-in action.That is, the ramp surfaces will interact to flex (e.g., elasticallydeform) the upper portion of the housing so the main body can snap intoposition. At this stage, locking cap 102 is inserted into the anchorhead and is in an open, unengaged or uncoupled position. Rod 108 isunlocked (i.e., unclamped); note space 1108 between rod 108 and saddle1100 in FIG. 24. Furthermore, anchor member 106 is also unlocked (i.e.,unclamped); note space 2508 between sleeve 612 and housing 710 in FIG.25. At this stage, collet 814 is not compressed against the head ofanchor member 106, thus allowing member 106 to axially rotate orangulate, as discussed above. Preferably the saddle is inserted into thetop opening 618 of the sleeve when the locking cap 102 is inserted intothe housing 710. More preferably, the dimensions of the saddle that fitwithin the opening 618 of the sleeve are larger than the opening 618 inits natural state such that the saddle flexes the stanchions 615 a,boutward. The tapered top inner surface 622 of the sleeve may facilitatethe spreading of the stanchions 615 a,b.

Locking cap 102 is next preferably rotated about 90° (i.e., about ¼turn) in the direction of arrow 2731 (see FIG. 27). Note that thelocking cap cannot be rotated in the opposite direction becausestructures 733 a,b on housing 710 (see FIGS. 7A and 27) would contacttabs 920 a,b on main body 900. As locking cap 102 is rotated in thedirection of arrow 2731, flexible resilient tabs 920 a,b encounterdimples 2730 a,b on housing 710 (see FIG. 27). Dimples 2730 a,bpreferably have respective beveled or ramp surfaces 2735 a,b tofacilitate moving tabs 920 a,b up and over respective dimples 2730 a,bin order to attach the locking cap in the closed position to anchor head104. The main body 900 does not have screw threads that may becross-threaded when attaching locking cap 102 to anchor head 104. Rampsurfaces 2737 a,b of dimples 2735 a,b are configured such that apredetermined torque RT is needed to slide and deflect tabs 920 a,b overthe ramp surfaces. The predetermined torque RT to overcome and slideover the dimples is preferably less than the predetermined set screwtorque ST (RT<ST) such that the torque applied to the set screw mayrotate both the set screw and main body together (i.e., the locking cap)into the closed position. In this exemplary embodiment, a singleinstrument or tool applied to the set screw can rotate the main bodyfrom the open to the closed positions. The main body is rotated in thedirection of arrow 2731 in FIG. 27 until the tabs pass straight-walledsurfaces 2737 a,b so that the tabs return to their unflexed position andrest in the recesses 2739 a,b formed between stop surfaces 2737 a,b andretention surfaces 2738 a,b. Preferably the height of the tabs 920 isapproximately the height of the dimples (e.g., straight wall stopsurface 2737) and the height of retention surface 2738 so that when tabs920 are in recesses 2739, the top surface of the tab is approximatelylevel with the top surface of structures 733. Surfaces 2737 a,b arepreferably straight-walled such that if the locking cap is rotated inthe opposite direction after sliding over the ramp surfaces, therespective tabs 920 a,b would abut against straight walled surfaces 2737a,b in the closed position. Surfaces 2737 a,b thus act as stops toprevent locking cap 102 from rotating backward (to the open position)unless sufficient force is applied. If the user continues to rotate thecap in the direction of arrow 2731 (FIG. 27), the tabs 920 a,b willcontact abutment surfaces 2738 a,b and resist, and more preferably,prevent further rotation of main body 900.

FIGS. 28-33 show locking cap 102 inserted into the anchor head and inthe closed or engaged or coupled position. As shown in FIG. 32, flanges3236 and 3238 on main body 900 are positioned in corresponding grooves2636 and 2638, respectively, on housing 710, and flange 3240 on housing710 is positioned in a corresponding groove 3241 on main body 900.Flange 3238 preferably has an inclined surface 3242 that tapers from adiameter approximately equal to the inside diameter of anchor head 104to the maximum diameter illustrated in FIG. 32. Flange 3238 may alsohave inclined surfaces 3222 and 3224 formed on their sides so that theflange 3238 increases in thickness. Flange 3236 preferably is similarlyconfigured to flange 3238. The inclined surfaces 3242, 3222 and 3224 mayfacilitate flange 3238 to enter the corresponding groove 2638.Preferably, flange 3238 and groove 2638 are dovetailed, as shown at area3242, to prevent any anchor head splaying that may occur. Morespecifically, the projection or flange 3238 has an angled surface 3242 aas shown in FIG. 32 that corresponds to angled surface 3242 b such thatif an outward force is applied to the anchor head 104, as a result, forexample, of clamping forces as the rod is clamped into position, surface3242 a of flange 3238 would contact surface 3242 b of flange 3240 andprevent anchor head 104 from expanding or splaying. Groove 2636preferably is similarly configured to groove 2638. At this stage, rod108 is still unlocked; again, note the space between rod 108 and saddle1100 in FIG. 30. Accordingly, as a comparison, FIGS. 31 and 25 show theabout 90° turn to set locking cap 102 in the closed position did notchange substantially, if at all, the relative positions of sleeve 612and collet 814. In this closed position, the tabs 920 interacting withthe stop surfaces 2737 prevent the locking cap from rotating to the openposition while the interaction of the flanges 3236, 3238 with grooves3236, 3238 prevent the locking cap from being lifted out of housing 710.

Advantageously, the same tool or driving mechanism used to insert andclose locking cap 102 in anchor head 104 can be used in a continuousaction to further rotate set screw 1000. The main body and set screwrotate together until the main body snaps into the closed position, atwhich time tabs 920 a,b contact structures 2738 a,b on top end 114 ofanchor head 104 of bone anchor 100. As tabs 920 a,b contact structures2738 a,b on anchor head 104, the torque to rotate the main body and setscrew increases until the torque applied is larger than thepredetermined set screw torque ST, at which point the frictionconnection holding the set screw to the main body is overcome and theset screw continues to rotate alone independent of the main body. Thesame tool can be advantageously used to move the set screw downward inanchor head 104 to the locked position as described below. In addition,the same instrument used to free the set screw from the main body, andwhich may also be used to move the set screw downward to the rod lockedposition where the rod is clamped, may also be used to loosen the setscrew (i.e., rotate the set screw in the opposite direction) until theflared thread portion 1925 of the set screw engages the main body, atwhich time, both the main body and set screw may rotate together. Ifsufficient force is applied in the direction opposite arrow 2731, thelocking cap can be returned to the open position. Note that the torquerequired to overcome the blockage of flared thread portion 1925 by themain body is preferably greater than the torque TT required for tabs 920a,b to deflect over the straight wall surfaces 2737 a,b.

Optionally, main body 900 may have a pair of oppositely positionedscalloped cutouts where straight sections 925 of main body 900 arelocated. The straight section 925 or scalloped cutouts may be used witha different tool for alternatively driving main body 900. Such a tool,however, may not be able to also drive set screw 1000.

FIGS. 34-38 show locking cap 102 inserted and closed in anchor head 104,and set screw 1000 driven downward in anchor head 104 to lock rod 108 inplace. As set screw 1000 is driven downward, it presses saddle 1100against rod 108 to press the rod into the lower portion of the sleeve612. Note the lack of space between saddle 1100 and rod 108 in FIG. 36.As the set screw is driven still further downward, the saddle pressesthe rod further downward which causes sleeve 612 to move downward,compressing collet 814 against the head of anchor member 106, lockingthe anchor member in place. More preferably, the sleeve 612 movesdownward in the housing body 710 such that its interior tapered surfaceinteracts with the tapered exterior surface of the collet 814 so thatthe resilient fingers of the collet are compressed inward to lock theanchor member into position so that the anchor member will be fixed tothe body 710. Set screw 1000 may be driven downward until the bottomsurface 2512 of sleeve 612 butts against the bottom inside surface 718of housing 710, or until the bottom surface 2512 butts against theshoulder 828 of the collet (shown in FIG. 37). The anchor may also beconfigured so that the bottom surface 2512 of the sleeve may not buttagainst either the housing or collet when the anchor member is in thelocked position. Additionally or alternatively, retention tabs 613 a,bmay butt against the respective lower edges 3015 a,b of slots 715 a,bupon completion of the anchor member locking process (as shown in FIG.36). Note that the rod and anchor member locking process (i.e., theoperation of moving of the set screw once it is free from the main bodyso that the saddle contacts the rod) has little, if any, effect on therelative closed positions of main body 900 and housing 710, as acomparison of FIGS. 32 and 38 shows.

A further illustrative method of using the bone fixation systemutilizing the bone anchor 100 is described below. The bone anchor 100including the anchor member and assembled body (including the sleeve andcollet but preferably without the locking cap) is attached to the bone.At this step, the anchor member 106 preferably may polyaxially rotate orangulate with respect to housing 710. The anchor member 106 may beprovisionally locked with respect to the housing 710 by applying a forceto move sleeve 612 down toward the bottom opening 415 to compress thecollet 814 against the head 717 to prevent movement of the anchor memberrelative to the housing 710. This provisional locking may be performedbefore the spinal rod is placed in the housing, or thereafter, and maybe performed before the locking cap is attached to the housing or thespinal rod is clamped into the housing. In one method, the spinal rodmay be placed in the sleeve and used to apply pressure on the sleeve tolock the anchor member to the housing before and independent of lockingthe spinal rod. With the anchor member locked in the housing 710, thespinal rod may be moved in the housing 710 (translated, rotated) orremoved therefrom. The spinal rod may thereafter be clamped to thehousing by using the locking cap 102 as described above.

Note that the locking cap is not limited to use in polyaxial boneanchors. FIGS. 39 and 40 show alternative bone anchors that can employthe locking cap of FIGS. 7 and 9-11. Bone anchor 3900 is an example of amonoaxial bone anchor having anchor member 3906. Bone anchor 4000 hasanchor member 4006, which is an example of a hook which may engage thespinous process, a lamina, or other bone.

FIGS. 2 and 3 illustrate tool engaging surfaces 140 and 150 on theexterior of anchor head 104 that may be used with holding andmanipulation instruments to assist in the implantation process.

A first illustrative method of fixation of the spine will be described.The method described below may be performed using any of the boneanchors described above, or any other bone anchors known in the art,although the polyaxial and monoaxial bone anchors described above arepreferred. The method generally includes attaching a first bone anchor100 to a vertebrae, preferably attaching a second bone anchor 100 toanother vertebra, and securing a spinal rod to the first and second boneanchors 100 to align the vertebrae. This may be accomplished, forexample, by inserting anchor member 106 of first bone anchor 100 into afirst vertebra. The second bone anchor 100 may alternatively beimplanted into one or more vertebrae in other regions of the spine(i.e., the cervical, thoracic, or lumbar regions).

In order to insert anchor member 106 into the vertebra, it may benecessary to insert anchor member 106 at an orientation of between about0° and about 25° medially or laterally, and more preferably betweenabout 0° and about 15° medially or laterally. Additionally oralternatively, it may be necessary to insert anchor member 106 at anorientation of between about 30° and about 50° upward, and between about30° and about 40° upward. The polyaxial bone anchors described above maybe configured and dimensioned to provide the necessary medial or lateraland/or upward angulation, although the present method is not limited tothe structures of polyaxial bone anchors described herein.

Prior to inserting anchor member 106, drilling and/or tapping a holeinto the vertebra may be desirable. In the case where the hole istapped, it may be preferable not to tap the anterior cortex of thevertebra. Anchor head 104 and curved head 717 preferably may bepreassembled before anchor member 106 is inserted into the vertebrae.Alternatively, once anchor member 106 has been fully inserted into thefirst vertebrae, anchor head 104 may be snapped onto the curved head 717of anchor member 106.

Second polyaxial anchor 100 is preferably attached to an adjacent orother vertebra, for example, by threading anchor member 106 (which inthis case is a screw) into the vertebra. Alternatively, second polyaxialanchor 100 may be attached to other vertebrae including those in thecervical, thoracic, or lumbar regions. Once the second polyaxial anchoris implanted, the first and second anchor heads 104 may be rotated toalign their respective rod-receiving channels so that a spinal rod 108may be inserted therein. Once the vertebrae have been repositioned tocorrect the deformity at hand, respective locking caps 102 of first andsecond polyaxial bone anchors 100 may be tightened to secure spinal rod108 to the first and second polyaxial bone anchors 100, and to fix theangular positions of the anchor heads 104 with respect to the anchormembers 106, thus forming a substantially rigid construct.

Alternatively, one end of spinal rod 108 can be inserted into one ofanchor heads 104, and spinal rod 108 manipulated to reposition thevertebral bodies. Then the other end of the spinal rod 108 can beinserted into the other of anchor heads 104 and then spinal rod 108fixed in position. The first end of spinal rod 108 may be fixed in oneof anchor heads 104 with the locking cap before spinal rod 108 ismanipulated to reposition the vertebral bodies. In yet anotherembodiment of this method, bone anchors 100 may be inserted into thespine as described above, both ends of spinal rod 108 may be insertedinto anchors 100, and one end of the spinal rod fixed or secured intothe anchor 100 and a distraction or compression force applied to movethe polyaxial anchor along spinal rod 108 to apply either a distractionor compression force, and thereafter fixing the second end of spinal rod108 into the bone anchor.

Once the first and second polyaxial anchors 100 have been implanted,their anchor heads 104 may rotated to align their respectiverod-receiving channels so that a spinal rod 108 may be inserted therein.Once the vertebrae have been repositioned to correct the deformity athand, the locking caps may be tightened to secure spinal rod 108 tofirst and second polyaxial anchors 100, and to fix the angular positionsof the anchor heads 104 with respect to anchor member 106, thus forminga substantially rigid construct.

While it is apparent that the illustrative embodiments of the inventiondisclosed herein fulfill the objectives stated above, it will beappreciated that numerous modifications and other embodiments may bedevised by those skilled in the art. Features and structures, such as,for example, the locking cap ca be used singularly or in combinationwith other features and structures. Therefore, it will be understoodthat the appended claims are intended to cover all such modificationsand embodiments which come within the spirit and scope of the invention.

1.-26. (canceled)
 27. A bone anchor for attaching a fixation element to a bone, the bone anchor comprising: an anchor member having a head portion and a bone engaging portion for attachment to the bone; an anchor head having a top portion, a bottom portion and a hollow interior bore extending from the top portion to the bottom portion, the head portion of the anchor member being receivable within the hollow interior bore of the anchor head such that the anchor member is polyaxially rotatable with respect to the anchor head, the anchor head including a generally U-shaped opening for receiving the fixation element; and a locking cap including a main body and a set screw, the main body being releasably engageable with the top portion of the anchor head, the set screw being friction fit and rotatable mounted to the main body in a preload condition, a predetermined torque being applied in a first direction to the set screw relative to the main body to rotate the set screw relative to the main body from the preload condition to a free condition.
 28. The bone anchor of claim 27, wherein the main body of the locking cap is secured to the top portion of the anchor head by rotating the main body approximately ninety degrees (90°) with respect to the anchor head, the main body being rotated from an open position to a closed position.
 29. The bone anchor of claim 28, wherein the main body of the locking cap includes at least one resiliently deflectable tab extending therefrom and the anchor head includes at least one dimple on its top surface, the tab deflecting over the at least one dimple upon rotation of the main body in the first direction with respect to the anchor head from the open position to the closed position.
 30. The bone anchor of claim 29, wherein the at least one dimple includes a stop surface, the stop surface resisting rotation of the main body relative to the anchor head in a second direction opposite the first direction when the main body is in the closed position.
 31. The bone anchor of claim 30, wherein the top surface of the anchor head further includes a retention surface, the stop surface and the retention surface defining a recess, the tab being located in the recess when the main body is in the closed position.
 32. The bone anchor of claim 27, further comprising a compressible collet having a bottom surface abutting a lower flange in the bottom portion of the anchor head.
 33. (canceled)
 34. The bone anchor of claim 35, wherein the sleeve has an internal recess, the recess receiving at least a portion of a collet therein in an assembled configuration, the sleeve including a tapered inner surface, the collet including a tapered outer surface, the tapered inner surface of the sleeve providing a compression force on the tapered outer surface of the collet in a locked position.
 35. The bone anchor of claim 27, wherein the fixation element is a spinal rod having an outer diameter and the bone anchor further comprises a sleeve having a U-shaped channel, the sleeve including opposing stanchions that define at least a portion of the U-shaped channel, the stanchions being flexible and defining a width therebetween at a top opening, the width being smaller than the outer diameter of the spinal rod so that the spinal rod is snap-fitted into the sleeve during insertion of the rod into the U-shaped channel.
 36. The bone anchor of claim 32, wherein the anchor head has an assembly groove facing the hollow interior bore and the collet has an extended shoulder, the assembly groove receiving the extended shoulder therein when the collet is located within the hollow interior bore as the head portion of the anchor member is inserted into an internal space of the collet.
 37. The bone anchor of claim 27, wherein the locking cap further includes a saddle coupled to the set screw, the saddle being rotatable with respect to the set screw, the saddle including an arcuate-shaped recess with grooves thereon to grip the fixation element in a locked position.
 38. (canceled)
 39. A bone anchor for attaching a fixation element to a bone, the bone anchor comprising: an anchor member having a head portion and a bone engaging portion for attachment to the bone; an anchor head receiving the head portion of the anchor member into a hollow interior bore so that the anchor member is polyaxially rotatable with respect to the anchor head, the anchor head includes a generally U-shaped opening for receiving the fixation element; and a locking cap a main body and a set screw, the main body engageable with a top portion of the anchor head, the set screw including external threads and a flared portion at a bottom end, the main body including internal threads that mate with the external threads of the set screw, the set screw being friction fit to the main body in a preload condition such that initial rotation of the set screw in a first direction causes the main body to rotate with the set screw until a predetermined torque is applied to the set screw relative to the main body, the flared portion engaging a bottom end of the main body in the preload condition.
 40. The bone anchor of claim 39, wherein the predetermined torque is about two (2) Newton-meters to about six (6) Newton-meters.
 41. A bone anchor for attaching a fixation element to a vertebrae, the bone anchor comprising: an anchor member for attachment to the vertebra; an anchor head having a top portion and a bottom portion, the anchor head having a generally tubular-shape bore an interior bore, a top opening in the top portion communicating with the interior bore and a bottom opening in the bottom portion communicating with the interior bore, the top portion having an end wall, at least one groove located proximate the top opening a U-shaped opening formed in the anchor head to receive the fixation element, the end wall including at least one dimple formed thereon; and a locking cap comprising a main body and a set screw, the main body includes at least one deflectable tab and at least one flange, the at least one deflectable tab interacting with the at least one dimple so that the at least one tab is deflected as the main body is rotated in a first direction with respect to the anchor head from an open position into a closed position the at least one flange positioned to in the at least one groove in the closed position provisionally retain the fixation element inside the U-shaped opening the dimple resisting rotation of the main body in a second direction opposite the first direction in the closed position.
 42. The bone anchor of claim 41, wherein the at least one dimple includes a ramped surface and a stop surface, the ramped surface being inclined with respect to the end wall and the stop surface extending substantially perpendicular from end wall.
 43. The bone anchor of claim 42, wherein the end wall further includes a retention surface, the retention surface, the ramped surface and the stop surface defining a recess, the at least one deflectable tab located in the recess when the main body is in the closed position, the locking cap being rotated approximately ninety degrees (90°) relative to the anchor head from the open position to the closed position to position the at least one deflectable tab in the recess.
 44. The bone anchor of claim 41, wherein the at least one flange has a dovetail-shape and the at least one groove has a complementary dovetail-shape, the dovetail-shaped flange and the dovetail-shaped groove limiting the top portion of the anchor head from expanding beyond a predetermined distance when the locking cap is mounted to the anchor head in the closed position, the dovetail-shaped flange and groove resisting movement of the locking cap along a longitudinal axis of the anchor head in the closed position.
 45. A bone anchor for attaching a fixation element to a bones the bone anchor comprising: an anchor member having a head portion and a bone engaging portion for attachment to the bone; an anchor head having a top portion, a bottom portion and a hollow interior bore extending from the top portion to the bottom portion, the head portion of the anchor member receivable within the hollow interior bore of the anchor head, the top portion of the anchor head including a generally U-shaped opening for receiving the fixation element, a longitudinal axis and a retaining surface facing the hollow interior bore proximate the top portion, the retaining surface being located a first distance from the longitudinal axis; and a locking cap removably engageable to the top portion of the anchor head, the locking cap including a retaining surface located a second distance from the longitudinal axis in an open position, the second distance being greater than the first distance, the retaining surfaces of the anchor head and the locking cap facilitating a snap lock of the locking cap relative to the anchor head as the locking cap is engaged with the anchor head in the open position.
 46. (canceled)
 47. (canceled)
 48. (canceled)
 49. (canceled)
 50. A combination bone anchor and fixation element for securing adjacent vertebra to each other, the combination comprising: an anchor member for mounting to the vertebra, the anchor member having a head portion and a bone engaging portion; an anchor head having a bottom portion, a top portion and a longitudinal axis, the top portion including an end wall, the anchor head coupled to the anchor member proximate the bottom portion, the anchor head including a hollow interior bore and a generally U-shaped opening for receiving the fixation element; a sleeve positioned within the hollow interior bore, the sleeve including opposing stanchions that define at least a portion of a U-shaped channel, the stanchions being flexible and defining a width therebetween at a top opening, the width being smaller than the outer diameter of the spinal rod so that the spinal rod is snap-fitted into the sleeve during insertion of the rod into the U-shaped channel and the U-shaped opening; and a locking cap removably mountable to the top portion of the anchor head, the locking cap blocking movement of the fixation element out of the anchor head along the longitudinal axis when the locking cap is in a closed position.
 51. A method of assembling a bone anchor of a spinal fixation device that includes a fixation element, the bone anchor including an anchor member having a bone engagement portion and a head portion an anchor head having a top opening, a bottom opening and a hollow interior bore in communication with the top and bottom openings and an assembly groove facing the hollow interior bore and a collet having an internal space, a bottom opening in communication with the internal space, a shoulder at the bottom opening and a top end, the method comprising the steps of: a) inserting the head portion of the anchor member through the bottom opening of the anchor head into the internal bore; b) aligning the shoulder of the collet with the assembly groove of the anchor head; c) inserting the head portion of the anchor member through the bottom opening of the collet while the collet is located in the interior bore of the anchor head and the shoulder is aligned with the assembly groove so that the bottom opening of the collet expands urging the shoulder of the collet into the assembly groove; d) urging the head portion into the internal space such that the bottom opening of the collet retracts to a natural state and the shoulder moves out of the assembly groove, the head portion of the anchor member being retained within the internal space between the top end and the bottom opening of the collet; and e) urging the collet and anchor member toward the bottom opening of the anchor head such that the shoulder of the collet is positioned proximate the bottom opening of the anchor head.
 52. The bone anchor of claim 27 wherein the main body includes internal threads and the set screw includes external threads that mate with the internal threads of the main body, the set screw further including a flared portion at a bottom end that engages a bottom end of the internal threads of the main body in the preload condition.
 53. The bone anchor of claim 27 wherein the set screw includes a flared portion at a bottom end that engages the main body in the preload condition, the flared portion generally preventing rotation of the set screw relative to the main body in a second direction, opposite the first direction, in the preload condition.
 54. The bone anchor of claim 27 wherein the anchor head, main body and set screw include top surfaces, the top surfaces of the anchor head, main body and set screw being generally coplanar in a closed position and a locked position, the top surface of the set screw being spaced from the top surface of the main body in the preload condition.
 55. The bone anchor of claim 39, wherein a required torque applied to the locking cap to rotate the locking cap from an open position to a closed position relative to the anchor head is less than the predetermined torque.
 56. The bone anchor of claim 45 wherein the locking cap includes a ramp surface proximate a bottom end and the anchor head includes a ramp surface proximate the top portion, the ramp surfaces of the locking cap and the anchor head facilitating the snap lock of the locking cap to the anchor head.
 57. The bone anchor of claim 55 wherein the locking cap includes a main body and a set screw, the set screw being threadably engaged with the main body, the retaining surface and the ramp surface of the locking cap being located on the main body. 